Two-photon absorption (TPA) phenomenon, theoretically predicted by Maria Goppert-Mayer in 1931 and experimentally observed by Kaiser and Garrett in 1965, has received little consideration for practical applications until the advent of high peak-power laser systems and the availability of materials with large two-photon absorption. However, from the mid 1990's to the present, significant advances have been made in the design and synthesis of two-photon absorbing materials with very large cross-section (σ2) values.
In the process of two-photon absorption (TPA), a transition from a molecule's ground state to an excited state occurs by simultaneous absorption of two photons which energetically sum up to the transition energy. Two photon absorption processes take place at photon energies which are outside of the absorption band corresponding to the linear, one photon absorption transition. Furthermore, the amount of two photon absorption varies quadratically with irradiation intensity, not linearly as with one one photon absorption. As a result, when a chromophore is irradiated at a typical two photon absorption wavelength, a high percentage of the incident intensity will be transmitted. However, at high intensities, as two photon absorption increases in strength, a lower percentage of the incident intensity is transmitted.
TPA-based applications at present include optical power limiting (1), up-conversion lasing (2), three dimensional fluorescence imaging (3), pulse reshaping and stabilization (4), and photodynamic therapy (5).
A current design strategy for the construction of two photon absorbing molecules is based on a three component, electronically conjugated system which electron donors (D) and electron acceptors (A). Both symmetrical and asymmetrical molecules can be designed according to this model, which have been proven to be effective to get compounds with large two-photon absorption. AF-50, a benchmark two-photon absorbing compound developed by Air Force lab, is an asymmetrical molecule. Perry and co-workers (6) have also discovered symmetrical structured chromophores with large two-photon absorption. Prasad and co-workers have developed multi-branched TPA chromophores and observed enhanced two-photon absorption.
Presently, most chromophores with efficient two-photon absorption (e.g., AF 350 and AF 389(7)) are highly fluorescent upon excitation by a strong laser beam. This feature is desirable for applications such as three dimensional fluorescence imaging, two-photon pumped lasing and two-photon biosensing. However, it is undesirable for some potential applications, including those in which fluorescence would interfere with the intended function of the material. For example, materials which fluoresce strongly are undesirable for use in optical power limiting materials which are intended for the protection of human eyes, sensors, etc. Fluorescence is also undesirable in those applications in which the energy imparted to a molecule by the two photon absorption is to be used in the performance of a function, such as, for example, two-photon induced photopolymerization or three-dimensional microfabrication. Thus, the development materials with high TPA cross section and quenched fluorescence would be welcomed in the art.
Recently, extensive efforts have been concentrated on the synthesis of π-conjugated push-pull nonlinear optical (NLO) chromophores and macroscopic assemblies, such as polymers or dendrimers(8). This rapid development is consistent with the increasing interest in chemically and thermally stable molecules with nonlinear optical or luminescent properties, such as TPA.